Method of semiconductor nanoparticle synthesis

ABSTRACT

A method is described for the manufacture of semiconductor nanoparticles. Improved yields are obtained by use of a reducing agent or oxygen reaction promoter.

CROSS REFERENCE To RELATED APPLICATIONS

[0001] This application claims priority under 35 U.S.C. § 119(e)(1) toU.S. Provisional Application Serial No. 60/326,746, filed Oct. 2, 2001;U.S. Provisional Application Serial No. 60/401,671, filed Aug. 6, 2002;and U.S. Provisional Application Serial No. 60/404,628, filed Aug. 19,2002; the disclosures of which are incorporated by reference herein intheir entireties.

FIELD OF THE INVENTION

[0002] This invention relates to nanoparticles. More particularly, theinvention relates to methods for making and using semiconductornanoparticles. The invention finds utility in a variety of fields,including biology, analytical and combinatorial chemistry, medicaldiagnostics, and genetic analysis.

BACKGROUND OF THE INVENTION

[0003] Semiconductor nanocrystals (also known as quantum dot particles)whose radii are smaller than the bulk exciton Bohr radius constitute aclass of materials intermediate between molecular and bulk forms ofmatter. Quantum confinement of both the electron and hole in all threedimensions leads to an increase in the effective band gap of thematerial with decreasing crystallite size. Consequently, both theoptical absorption and emission of semiconductor nanocrystals shift tothe blue (higher energies) as the size of the nanocrystals gets smaller.

[0004] Semiconductor nanocrystals are nanoparticles composed of aninorganic, crystalline semiconductive material and have uniquephotophysical, photochemical and nonlinear optical properties arisingfrom quantum size effects, and have therefore attracted a great deal ofattention for their potential applicability in a variety of contexts,e.g., as detectable labels in biological applications, and as usefulmaterials in the areas of photocatalysis, charge transfer devices, andanalytical chemistry. As a result of the increasing interest insemiconductor nanocrystals, there is now a fairly substantial body ofliterature pertaining to methods for manufacturing such nanocrystals.

[0005] In general, these routes can be classified as involvingpreparation in glasses (Ekimov et al., JETP Letters 34:345 (1981));aqueous preparation, including preparations that involve use of inversemicelles, zeolites, Langmuir-Blodgett films, and chelating polymers(Fendler et al., J. Chem. Society, Chemical Communications 90:90 (1984)and Henglein et al., Ber. Bunsenges. Phys. Chem. 88:969 (1984)); andhigh temperature pyrolysis of organometallic semiconductor precursormaterials, i.e., rapid injection of precursors into a hot coordinatingsolvent (Murray et al., J. Am. Chem. Soc. 115:8706 (1993) and Katari etal., J. Phys. Chem. 98:4109 (1994)). The two former methods yieldparticles that have unacceptably low quantum yields for mostapplications, a high degree of polydispersity, poor colloidal stability,a high degree of internal defects, and poorly passivated surface trapsites. In addition, nanocrystals made by the first route are physicallyconfined to a glass matrix and cannot be further processed aftersynthesis.

[0006] Improved synthesis conditions have been reported that utilizecadmium salts (Peng, et al., J. Am. Chem. Soc. 123:183-184 (2001)).These conditions provide certain advantages over the rapid injectionmethod. The use of cadmium acetate, cadmium oxide or other such Cd(II)salts, pre-complexed with a ligand such as tetradecylphosphonic acidprovides for a cadmium precursor that is particularly suitable fornanocrystal synthesis. These reactions have numerous desirable features,including improved safety and relatively wide tolerance for productionvariables such as precursor injection rate and temperature. Ofparticular note is that these reactions can be tuned to yield verynarrow photoluminescence spectra over a wide range of usefulwavelengths. Unfortunately, it is difficult to optimize the particleyield, while maintaining the desirable features of the Cd(II) synthesisconditions. In particular, for smaller size nanoparticle synthesis,yields have been very poor under Cd(II) synthesis conditions. Reactionconditions that provide such low yields are not only more expensive toimplement on a manufacturing scale, but they often require much largerreactors and produce more hazardous waste.

[0007] Thus, there remains a need in the art for improved methods formanufacturing nanoparticles, and smaller nanoparticles in particular.Such methods would ideally provide a high product yield of internallydefect free, high band edge luminescence nanoparticles with no orminimal trapped emission. Such methods would also ideally provide forthe manufacture of particles that exhibit near monodispersity and have arelatively narrow particle size distribution. Finally, such methodswould be useful not only with semiconductor nanoparticles, but also withother types of nanoparticles, e.g., semiconductive nanoparticles thatare not necessarily crystalline and metallic nanoparticles.

[0008] The present invention addresses those needs by providing improvedmethods for manufacturing nanoparticles. By controlling the nucleationdensity the methods of the invention provide for a predictable andcontrollable final particle size, as well as many of the aforementionedproperties.

SUMMARY OF THE INVENTION

[0009] One aspect of the invention relates to a method of producingnanoparticles comprising: (a) mixing a first precursor and at least onecoordinating solvent to form a first mixture; (b) exposing the firstmixture to a reaction promoter selected from the group consisting ofoxygen and a reducing agent; (c) heating the first mixture to atemperature that is sufficiently high to form nanoparticles (nucleationcrystals) when a second precursor is added; (d) introducing a secondprecursor into the first mixture to form a second mixture therebyresulting in the formation of a plurality of nanoparticles; and (e)cooling the second mixture to stop further growth of the nanoparticles.

[0010] Another embodiment of the invention is a method of producingnanoparticles comprising: (a) mixing a first precursor and a secondprecursor with at least one coordinating solvent to form a firstmixture; (b) heating the first mixture to a temperature that issufficiently high to form nanoparticles when a reaction promoter isadded; (c) exposing the first mixture to a reaction promoter, thereaction promoter being selected from the group consisting of oxygen anda reducing agent, to form a second mixture thereby resulting in theformation of a plurality of nanoparticles; and (d) cooling the secondmixture to stop further growth of the nanoparticles.

[0011] Yet another embodiment of the invention is a method of producingthe nanoparticle shell comprising (a) mixing nanoparticles with at leastone coordinating solvent to form a first mixture; (b) heating the firstmixture to a temperature that is sufficiently high to form a shell onthe nanoparticles when first and second precursors are added; (c)introducing first and second precursors into the first mixture to form asecond mixture thereby resulting in the formation of shells on aplurality of nanoparticles; and (d) cooling the second mixture to stopfurther growth of the shell; wherein the method further comprisesexposing the first or second mixture to a reaction promoter selectedfrom the group consisting of oxygen and a reducing agent. Thenanoparticles can be produced by the methods described herein or can beproduced by any method known in the art.

[0012] Another aspect of the invention pertains to a method of producingsemiconductive nanoparticles having a valency “n”, comprising: (a)mixing a first precursor having a valency “c” and at least onecoordinating solvent to form a mixture; (b) exposing the mixture to areaction promoter wherein the reaction promoter converts the valency ofthe first precursor to a valency “a”; (c) heating the mixture to atemperature that is sufficiently high to form nanoparticles when asecond precursor is added; (d) introducing a second precursors into themixture to form a second mixture thereby resulting in the formation of aplurality of nanoparticles; wherein the second precursor has a valency“b”, and wherein a+b=n and c+b≠n; and (e) cooling the second mixture tostop further growth of the nanoparticles.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 and FIG. 2 are graphical representations of the effect ontemporal wavelength evolution during the course of a CdSe nanocrystalcore-forming reaction as a function of added reaction promoter, asdescribed in Examples 1 and 2. FIG. I shows the effect on emission peakwavelength, while FIG. 2 shows the effect on the full peak width at halfmaximum (FWHM).

[0014]FIG. 3 and FIG. 4 are graphical representations of the effect ofincreasing amounts of diphenylphosphine (DPP) on emission peakwavelength and FWHM as described in Example 1.

[0015]FIG. 5 is a graphical representation of the particle countsrecovered from each of the reactions shown in the FIG. 2 and illustratesthe level of control afforded by the use of reaction promoters.

[0016]FIG. 6 is a graphical representation of the effect of the time ofair exposure prior to the TOPSe injection as described in Example 3. Thescale on the left-hand coordinate is the emission peak wavelength innanometers and the right-hand ordinate is the FWHM peak height innanometers.

[0017]FIG. 7 is a graphical representation of the effect of a spike ofair-exposed TOP before the TOPSe injection as described in Example 5.

[0018]FIG. 8 is an illustration of one implementation of anelectrochemical system which could be used to prepare nanocrystals bythe methods described here.

DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention provides an improved method of producingnanoparticles having a means for controlling the reactivity of asolution of nanoparticle precursors through introduction of a reactionpromoter prior to the nucleation of nanoparticles. This control ofreactivity allows for control of the relative number of nanoparticlenuclei formed in a single nucleation period. Control of nucleationdensity provides for a predictable and controllable final nanoparticlesize, size distribution and yield. In addition, the method also providesfor control of the growth rate once nucleation occurs, which allows forcontrol of size focusing, resulting in very narrow size distributions.The methods described herein can be used to form the nanoparticle core,the nanoparticle shell, or both. In addition, the methods describedherein can be used to form nanoparticles that are smaller than has beenpossible using traditional methods known in the art, e.g., for preparingCdSe nanoparticles which emit with an emission maximum in the range of400 nm to 500 nm.

[0020] In general, the invention provides a method of producingnanoparticles by contacting a first precursor M′ (valency=c) with areaction promoter, wherein the reaction promoter converts M′ to M(valency=a); and then contacting the M precursor with a second precursorX (valency=b) to produce nanoparticles having a valency “n”, whereinc+b≠n and a+b=n.

[0021] Either the first elemental component or the second elementalcomponent of the nanoparticle can gain electrons. Therefore, theresulting nanoparticle can have a composition of MX or XM, asexemplified below. For example, the method of the invention can be usedfor the production of CdSe nanoparticles, i.e., MX nanoparticles wheren=0. The first precursor M′ can be Cd⁺² (c=⁺2). Upon reaction with areaction promoter of the invention, such as DPP or hydroquinone, Cd⁺² isconverted or reduced to Cd⁰ (M, where a=0). This is then contacted withthe second precursor Se⁰ (X, where b=0) to produce the CdSenanoparticles having a valency of 0.

Cd⁺²+reaction promoter→Cd⁰

Cd⁰+Se⁰→CdSe

[0022] The method of the invention can also be used for the productionof, for example, InP nanoparticles, i.e., XM nanoparticles where n=0.The first precursor M′ can be P⁰ (c=0). Upon reaction with a reactionpromoter of the invention, such as DPP or hydroquinone, P⁰ is convertedor reduced to P⁻³ (M, where a=⁻3). This is then contacted with thesecond precursor In⁺³ (X, where b=⁺3) to produce the InP nanoparticleshaving a valency of 0.

P⁰+reaction promoter→P⁻³

In⁺³+P⁻³→InP

[0023] Accordingly, one embodiment of the invention is a method ofproducing semiconductive nanoparticles having a valency “n”, comprising:(a) mixing a first precursor having a valency “c” and at least onecoordinating solvent to form a first mixture; (b) exposing the firstmixture to a reaction promoter wherein the reaction promoter convertsthe valency of the first precursor to a valency “a”; (c) heating thefirst mixture to a temperature that is sufficiently high to formnanoparticles when a second precursor is added; (d) introducing a secondprecursor into the mixture to form a second mixture thereby resulting inthe formation of a plurality of nanoparticles; wherein the secondprecursor has a valency “b”, and wherein a+b=n and c+b≠n; and (e)cooling the second mixture to stop further growth of the nanoparticles.

[0024] More specifically, in one embodiment of the invention, the methodof producing nanoparticles is a one-pot synthesis technique thatinvolves: (a) mixing a first precursor, an optional ligand and at leastone coordinating solvent to form a first mixture; (b) exposing the firstmixture to a reaction promoter; (c) heating the first mixture to atemperature that is sufficiently high to form nanoparticles, i.e.,nucleation crystals, when a second precursor is added; (d) introducing asecond precursor into the first mixture to form a second mixture therebyresulting in the formation of a plurality of nanoparticles; and (e)cooling the second mixture to stop further growth of the nanoparticles.A reducing agent or oxygen source serves as the reaction promoter.

[0025] The exposure to a reaction promoter provides for a higher yieldof nanoparticles, typically up to three times more yield, morepreferably up to 19 times better yield when compared to those methodswhere no reaction promoter is used. In addition, the nanoparticle yieldcan be modulated by modulating the amount of reaction promoter added forthe length of time of exposure.

[0026] The methods described herein can provide nanoparticles having anaverage particle diameter within the range of about 1.5 to 15 Å, with aparticle size deviation of less that about 10% rms in diameter.

[0027] The methods described herein are particularly useful forpreparing a monodisperse population of CdSe nanoparticles having anemission peak wavelength that is preferably less than about 570 nm,preferably less than about 520 nm, more preferably less than about 500nm.

[0028] In addition, the methods described herein can provide amonodisperse population of nanoparticles having an emission peakwavelength that is less than about 35 nm at full width at half max(FWHM), preferably less than about 30 nm FWHM, more preferably less thanabout 25 nm FWHM.

[0029] I. Definitions and Nomenclature

[0030] Before describing detailed embodiments of the invention, it is tobe understood that unless otherwise indicated, this invention is notlimited to specific nanoparticle materials or manufacturing processes,as such may vary. It may be useful to set forth definitions that areused in describing the invention. The definitions set forth apply onlyto the terms as they are used in this patent and may not be applicableto the same terms as used elsewhere, for example in scientificliterature or other patents or applications including other applicationsby these inventors or assigned to common owners. The followingdescription of the preferred embodiments and examples are provided byway of explanation and illustration, and is not intended to be limiting.

[0031] It must be noted that, as used in this specification and theappended claims, the singular forms “a”, “an” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to “a nanoparticle” includes a single nanoparticle aswell as two or more nanoparticles, and the like.

[0032] In describing and claiming the present invention, the followingterminology will be used in accordance with the definitions set outbelow.

[0033] The term “nanoparticle” refers to a particle, generally asemiconductive particle, having a diameter in the range of about 1-1000nm, preferably in the range of about 2-50 nm, more preferably in therange of about 2-20 nm.

[0034] The terms “semiconductor nanoparticle” and “semiconductivenanoparticle” refer to a nanoparticle as defined herein, that iscomposed of an inorganic semiconductive material, an alloy or othermixture of inorganic semiconductive materials, an organic semiconductivematerial, or an inorganic or organic semiconductive core containedwithin one or more semiconductive overcoat layers.

[0035] The terms “semiconductor nanocrystal,” “quantum dot” and “Qdot™nanocrystal” are used interchangeably herein to refer to semiconductornanoparticles composed of an inorganic crystalline material that isluminescent (i.e., they are capable of emitting electromagneticradiation upon excitation), and include an inner core of one or morefirst semiconductor materials that is optionally contained within anovercoating or “shell” of a second inorganic material. A semiconductornanocrystal core surrounded by an inorganic shell is referred to as a“core/shell” semiconductor nanocrystal. The surrounding shell materialwill preferably have a bandgap energy that is larger than the bandgapenergy of the core material and may be chosen to have an atomic spacingclose to that of the core substrate.

[0036] The term “solid solution” is used herein to refer to acompositional variation that is the result of the replacement of ions orionic groups with other ions or ionic groups, e.g., CdS in which some ofthe Cd atoms have been replaced with Zn. This is in contrast to a“mixture,” a subset of which is an “alloy,” which is used herein torefer to a class of matter with definite properties whose members arecomposed of two or more substances, each retaining its own identifyingproperties.

[0037] By “luminescence” is meant the process of emittingelectromagnetic radiation (light) from an object. Luminescence resultswhen a system undergoes a transition from an excited state to a lowerenergy state with a corresponding release of energy in the form of aphoton. These energy states can be electronic, vibrational, rotational,or any combination thereof. The transition responsible for luminescencecan be stimulated through the release of energy stored in the systemchemically or added to the system from an external source. The externalsource of energy can be of a variety of types including chemical,thermal, electrical, magnetic, electromagnetic, and physical, or anyother type of energy source capable of causing a system to be excitedinto a state higher in energy than the ground state. For example, asystem can be excited by absorbing a photon of light, by being placed inan electrical field, or through a chemical oxidation-reduction reaction.The energy of the photons emitted during luminescence can be in a rangefrom low-energy microwave radiation to high-energy x-ray radiation.Typically, luminescence refers to photons in the range from UV to IRradiation, and usually refers to visible electromagnetic radiation(i.e., light).

[0038] The term “monodisperse” refers to a population of particles(e.g., a colloidal system) wherein the particles have substantiallyidentical size and shape. For the purpose of the present invention, a“monodisperse” population of particles means that at least about 60% ofthe particles, preferably about 75-90% of the particles, fall within aspecified particle size range. A population of monodisperse particlesdeviates less than 10% rms (root-mean-square) in diameter and preferablyless than 5% rms.

[0039] The phrase “one or more sizes of nanoparticles” is usedsynonymously with the phrase “one or more particle size distributions ofnanoparticles.” One of ordinary skill in the art will realize thatparticular sizes of nanoparticles such as semiconductor nanocrystals areactually obtained as particle size distributions.

[0040] By use of the term “narrow wavelength band” or “narrow spectrallinewidth” with regard to the electromagnetic radiation emission of thesemiconductor nanocrystal is meant a wavelength band of emissions notexceeding about 60 nm, and preferably not exceeding about 30 nm inwidth, more preferably not exceeding about 20 nm in width, and symmetricabout the center. It should be noted that the bandwidths referred to aredetermined from measurement of the full width of the emissions at halfpeak height (FWHM), and are appropriate in the emission range of200-2000 nm.

[0041] By use of the term “a broad wavelength band,” with regard to theexcitation of the semiconductor nanocrystal is meant absorption ofradiation having a wavelength equal to, or shorter than, the wavelengthof the onset radiation (the onset radiation is understood to be thelongest wavelength (lowest energy) radiation capable of being absorbedby the semiconductor nanocrystal). This onset occurs near to, but atslightly higher energy than the “narrow wavelength band” of theemission. This is in contrast to the “narrow absorption band” of dyemolecules, which occurs near the emission peak on the high energy side,but drops off rapidly away from that wavelength and is often negligibleat wavelengths further than 100 nm from the emission.

[0042] The term “emission peak” refers to the wavelength of light withinthe characteristic emission spectra exhibited by a particularsemiconductor nanocrystal size distribution that demonstrates thehighest relative intensity.

[0043] The term “alkyl” as used herein refers to a branched orunbranched saturated hydrocarbon group of 1 to approximately 24 carbonatoms, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl,t-butyl, octyl, decyl, tetradecyl, hexadecyl, eicosyl and tetracosyl, aswell as cycloalkyl groups such as cyclopentyl and cyclohexyl. Similarly,alkanes are saturated hydrocarbon compounds such as methane, ethane, andso forth. The term “lower alkyl” is intended to mean an alkyl group of 1to 4 carbon atoms, and thus includes methyl, ethyl, n-propyl, isopropyl,n-butyl, isobutyl and t-butyl.

[0044] The term “alkene” as used herein refers to a branched orunbranched hydrocarbon compound typically although not necessarilycontaining 2 to about 24 carbon atoms and at least one double bond, suchas ethylene, n-propylene, isopropylene, butene, butylene, propylene,octene, decylene, and the like. Generally, although not necessarily, thealkenes used herein contain 2 to about 29 carbon atoms, preferably about8 to about 20 carbon atoms. The term “lower alkene” is intended to meanan alkene of 2 to 4 carbon atoms.

[0045] The term “alkyne” as used herein refers to a branched orunbranched hydrocarbon group typically although not necessarilycontaining 2 to about 24 carbon atoms and at least one triple bond, suchas acetylene, allylene, ethyl acetylene, octynyl, decynyl, and the like.Generally, although again not necessarily, the alkynes used hereincontain 2 to about 12 carbon atoms. The term “lower alkyne” intends analkyne of 2 to 4 carbon atoms, preferably 3 or 4 carbon atoms.

[0046] II. Precursors

[0047] There are numerous inorganic materials that are suitable for useas materials for the core and/or shell of semiconductor nanoparticles.These include, by way of illustration and not limitation, materialscomprised of a first element selected from Groups 2 and 12 of thePeriodic Table of the Elements and a second element selected from Group16 (e.g., ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, MgS, MgSe,MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, and the like);materials comprised of a first element selected from Group 13 of thePeriodic Table of the Elements and a second element selected from Group15 (GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, and the like); ternaryand quaternary mixtures comprised of a Group 14 element (Ge, Si, and thelike); materials comprised of a first element selected from Group 14element of the Periodic Table of the Elements and a second elementselected from Group 16 (e.g., PbS, PbSe and the like); materialscomprised of a first element selected from Group 13 of the PeriodicTable of the Elements and a second element selected from Groups 15 and16 (e.g., AlS, AlP, AlSb, and the like); and alloys and mixturesthereof. As used herein, all reference to the Periodic Table of theElements and groups thereof is to the new IUPAC system for numberingelement groups, as set forth in the Handbook of Chemistry and Physics,81^(st) Edition (CRC Press, 2000).

[0048] The selection of the composition of the semiconductornanoparticle affects the characteristic spectral emission wavelength ofthe semiconductor nanocrystal. Thus, as one of ordinary skill in the artwill realize, a particular composition of a nanoparticle of theinvention will be selected based upon the spectral region beingmonitored. For example, semiconductor nanocrystals that emit energy inthe visible range include, but are not limited to, CdS, CdSe, CdTe,ZnSe, ZnTe, GaP, and GaAs. Semiconductor nanocrystals that emit energyin the near IR range include, but are not limited to, InP, InAs, InSb,PbS, and PbSe. Finally, semiconductor nanocrystals that emit energy inthe blue to near-ultraviolet include, but are not limited to, ZnS andGaN.

[0049] Precursors useful as the “first” precursor in the methods of theinvention include compounds containing elements from Groups 2 and 12 ofthe Periodic Table of the Elements (e.g., Zn, Cd, Hg, Mg, Ca, Sr, Ba,and the like), compounds containing elements from Group 13 of thePeriodic Table of the Elements (Al, Ga, In, and the like), and compoundscontaining elements from Group 14 of the Periodic Table of the Elements(Si, Ge, Pb, and the like).

[0050] Precursors useful as the “second” precursor in the methods of theinvention include compounds containing elements from Group 16 of thePeriodic Table of the Elements (e.g., S, Se, Te, and the like),compounds containing elements from Group 15 of the Periodic Table of theElements (N, P, As, Sb, and the like), and compounds containing elementsfrom Group 14 of the Periodic Table of the Elements (Ge, Si, and thelike).

[0051] Many forms of the precursors can be used in the methods of theinvention. Suitable element-containing compounds useful as the firstprecursor, can be organometallic compounds such as Cd(CH₃)₂, oxides suchas CdO, halogenated compounds such as CdCl₂, and other salts such ascadmium acetate.

[0052] Suitable second precursors include tri-n-alkylphosphine adductssuch as tri-n-(butylphosphine)selenide (TBPSe) andtri-n-(octylphosphine)selenide (TOPSe), hydrogenated compounds such asH₂Se, silyl compounds such as bis(trimethylsilyl)selenium ((TMS)₂Se),and metal salts such as NaHSe. These are typically formed by combining adesired element, such as Se, with an appropriate coordinating solvent,e.g., TOP. Other exemplary organic precursors are described in U.S. Pat.Nos. 6,207,299 and 6,322,901 to Bawendi et al., and synthesis methodsusing weak acids as precursor materials are disclosed by Qu et al.,(2001) “Alternative Routes toward High Quality CdSe Nanocrystals,” NanoLett., 1(6):333-337, the disclosures of which are incorporated herein byreference.

[0053] Both the first and the secondary precursor can be combined withan appropriate coordinating solvent to form a solution for use in themethods of the invention. The coordinating solvent used to form a firstprecursor solution may be the same or different from that used to form asecond precursor solution.

[0054] III. Coordinating Solvent

[0055] Suitable coordinating reaction solvents include, by way ofillustration and not limitation, amines, alkyl phosphines, alkylphosphine oxides, fatty acids, ethers, furans, phospho-acids, pyridines,alkenes, alkynes and combinations thereof. The solvent may actuallycomprise a mixture of solvents, often referred to in the art as a“solvent system”. Furthermore, the coordinating solvent might be amixture of an essentially non-coordinating solvent such as an alkane anda ligand as defined below.

[0056] Suitable amines include, but are not limited to, alkylamines sucha dodecylamine and hexyldecylamine, and so forth.

[0057] Exemplary alkyl phosphines include, but are not limited to, thetrialkyl phosphines, tri-n-butylphosphine (TBP), tri-n-octylphosphine(TOP), and so forth.

[0058] Suitable alkyl phosphine oxides include, but are not limited to,the trialkyl phosphine oxide, tri-n-octylphosphine oxide (TOPO), and soforth.

[0059] Exemplary fatty acids include, but are not limited to, stearicand lauric acids. It will be appreciated that the rate of nanocrystalgrowth generally increases as the length of the fatty acid chaindecreases.

[0060] Exemplary ethers and furans include, but are not limited to,tetrahydrofuran and its methylated forms, glymes, and so forth.

[0061] Suitable phospho-acids include, but are not limited tohexylphosphonic acid, tetradecylphosphonic acid, and octylphosphinicacid, and are preferably used in combination with an alkyl phosphineoxide such as TOPO.

[0062] Exemplary pyridines include, but are not limited to, pyridine,alkylated pyridines, nicotinic acid, and so forth.

[0063] Coordinating solvents can be used alone or in combination.TOP-TOPO solvent systems are commonly utilized in the art, as are otherrelated (e.g., butyl) systems. For example, TOP and TOPO can be used incombination to form a cadmium solution, while TOP, alone, can be used toform a selenium solution.

[0064] Technical grade coordinating solvents can be used, and benefitscan be obtained from the existence of beneficial impurities in suchsolvents, e.g. TOP, TOPO or both. However, in one preferred embodiment,the coordinating solvent is pure. Typically this means that thecoordinating solvent contains less than 10 vol %, and more preferablyless than 5 vol % of impurities that can function as reductants.Therefore, solvents such as TOPO at 90% or 97% purity and TOP at 90%purity are particularly well suited for use in the methods of theinvention.

[0065] IV. Ligand

[0066] In one preferred embodiment, ligands are included in thereaction. Ligands are compounds that complex with a precursor and/or ananoparticle. Suitable ligands include, by way of illustration and notlimitation, phospho-acids such as hexylphosphonic acid andtetradecylphosphonic acid, carboxylic acids such as isomers ofoctadecanoic acid, amines, amides, alcohols, ethers, alkenes, andalkynes. In some cases, the ligand and the solvent can be the same.

[0067] V. Reaction Promoter

[0068] The methods of the invention, to some extent, are based upon thepremise that particle growth kinetics are strongly impacted by theeffectiveness of the initial nucleation events and that the chemicalreduction of one of the precursors is expected to be an importantrate-determining factor. This is in contrast with state of the artmethodologies that operate under the assumption that precursor/particlesequestration events and the precursor-injection temperature dropdominates the nucleation/growth temporal interface.

[0069] The reaction promoter increases the reactivity of thenanoparticle precursors in such a way so as to allow control of thenucleation process, or growth process, or both. In the methods of theinvention, the reactants are exposed to the reaction promoter in acarefully controlled manner, typically by physically adding the reactionpromoter to the mixture. This serves to provoke and modulate increasedreactivity.

[0070] In the fast kinetic growth regime, nanoparticles can grow rapidlywhen the concentration of monomer precursors is high relative to thenumber of particles. Such growth is accompanied by narrowing of theparticle size distribution. As long as this condition exists, theparticle size distribution can remain focused. When the monomerconcentration is reduced to a level that cannot maintain the optimumgrowth rate, statistical broadening of size distributions is generallyobserved. Optimally, the reaction should be stopped prior to theoccurrence of such defocusing so as to ensure optimally narrow particlesize distributions. The methods of the invention achieve this bycontrolling the number of nuclei formed. The initial reactivity iscontrolled to make fewer nuclei and to obtain larger particles. Whensmaller particles are desired, reactivity can be boosted to produce morenuclei. In either case, the reaction is stopped at the point wheregrowth begins to slow due to monomer depletion. This corresponds to themaximum practical yield for the chosen particle size, while notsacrificing narrow particle size distribution.

[0071] The reaction promoter can be an oxygen source or a reducingagent. While not wishing to be bound by theory, there are several waysin which the oxygen reaction promoter, for example, may be functioningin the methods described herein. First, in the presence of O₂, theinitial nuclei in the core reaction may not be CdSe, but rather CdO (orCdOH). These nuclei form more easily than the CdSe nuclei for a varietyof reasons, such as issues of driving force, activation, anddifferential sequestration of precursors. These cores can provide agrowth site for CdSe. During the process, the oxygen atoms can beannealed out or can remain at the core of the final material. Second,some impurities may be present in the reaction, for example from theTOP. These impurities may hinder the growth of particles throughsequestration of redox reactivity. In this case, the oxygen in thereaction may be responsible for destroying the impurities and thusindirectly facilitating the reaction.

[0072] It is preferable to use two reducing equivalents per precursor(e.g., cadmium) equivalent to prepare the nanoparticles when saltfeedstocks are utilized (or created in situ through, for exampleacid-base reactions). Oxygen may facilitate these redox reactionsdirectly or indirectly through the formation of some intermediates. Anexample of this indirect mechanism involves initial oxidation of TOP toa species such as di-octyl-octylphosphinate. This species mightdisproportionate to form di-octylphosphine and octyl-di-octylphosphonateby the following scheme:

[0073] more oxidized products

[0074] Similar chemistries are possible with other oxidized impuritiessuch as di-octylphosphine oxide. The resulting secondary (or primary)phosphines are potent reductants that would be expected to enhance thereaction rates as observed. This last mechanism supports the directaddition of such phosphines as reducing agents as described below.Addition of oxidants and/or proton carriers other than oxygen/water canprovide an even more serviceable approach to taking full advantage ofthis effect.

[0075]FIG. 1 and FIG. 2 show the effect on temporal wavelength evolutionduring the course of a CdSe nanocrystals core-forming reaction as afunction of added reaction promoter. The line labeled “Control” is thestandard reaction with no added reaction promoter. The line labeled “Airadded” shows the effects of exposure of the reaction to air on peakemission wavelength and emission FWHM. The remaining three lines depictthe reaction course when equivalent amounts of reducing agent reactionpromoters are used, specifically, dicyclohexylphosphine, DPP, andhydroquinone. FIG. 1 demonstrates that both an oxygen source as well asreducing agents are suitable for use as reaction promoters in themethods of the invention. The key characteristics of these plots are thelength of the pre-nucleation induction, the initial nuclei size, thestall wavelength for growth, and the evolution of the degree ofdispersity (as estimated by emission FWHM).

[0076]FIG. 3 and FIG. 4 illustrate the use of increasing amounts of thereaction promoter, DPP. The concentrations are given as number ofequivalents relative to cadmium added to the core reactions. The amountof reaction promoter that is added can be used to tune the properties ofthe reactions including the yield of particles.

[0077]FIG. 5 illustrates the particle counts recovered from each of thereactions shown in FIG. 3 and illustrates the level of control affordedby the addition of reaction promoters.

[0078] 1. Reducing Agents

[0079] The reducing agent functions to provide electrons (reducingequivalents) to the reactants or reactant mixture. Phosphine-basedreductants are a preferred class of reducing agents for use in themethods of the invention. However, non-phosphine, non-ligating chemicalreductants such as hydroquinone are also suitable to provide thenecessary reducing equivalents. Accordingly, suitable reducing agentsinclude, by way of illustration and not limitation, chemical compoundssuch as tertiary, secondary, and primary phosphines (e.g.,diphenylphosphine, dicyclohexylphosphine, and dioctylphosphine); amines(e.g., decyl- and hexadecylamine); hydrazines; hydroxyphenyl compounds(e.g., hydroquinone and phenol); hydrogen; hydrides (e.g., sodiumborohydride, sodium hydride and lithium aluminum hydride); metals (e.g.,mercury and potassium); boranes (e.g., THF:BH₃ and B₂H₆); aldehydes(e.g., benzaldehyde and butyraldehyde); alcohols and thiols (e.g.,ethanol and thioethanol); reducing halides (e.g., I⁻ and I₃ ⁻);polyfunctional reductant versions of these species (e.g., a singlechemical species that contains more than one reductant moiety, eachreductant moiety having the same or different reducing capacity, such astris-(hydroxypropyl)phosphine and ethanolamine); and so forth.

[0080] In addition, it is expected that there may be particularadvantages associated with the use of an electrochemical system(cathode-anode system) as the reducing agent, i.e., the cathode wouldserve as a source of electrons. By utilizing an electrode as a source ofreducing equivalents, coulombic equivalents can be readily counted andtheir rate of delivery directly controlled. Use of electrodes alsoallows for controlling both the physical localization of reductionevents, as well as the potential for direct formation of particle arraysat the electrode surface. Since the cathode will be positioned withinthe reaction chamber, the material selection is preferably one that willnot react with the precursors, ligand or coordinating solvents. Theanode, will typically be positioned outside of the reaction vessel somaterial selection is not limited and any well known anode material canbe used. Exemplary cathode materials include platinum, silver, orcarbon. An exemplary method for delivering reducing equivalents to thecathode includes the use of a constant current or potentiostat in a two-(working and counter) or three-electrode (working, counter, andreference) configuration.

[0081] 2. Oxygen

[0082] The reaction promoter can also be any source of oxygen. In oneembodiment of the invention, the reaction promoter is an air stream,preferably dry. In this embodiment, the mixture is exposed directly tothe oxygen or oxygen source.

[0083] The oxygen can also be formed in situ. Accordingly, in anotherembodiment, the precursors are exposed to compounds which provide thesame effect as the aforementioned controlled exposure to air. Forexample, oxygen can be formed by a redox reaction. Therefore, since themechanism by which oxygen enhances the process could include a reductionstep, reducing agents can added directly to utilize redox reactivity.Suitable reducing agents are those described above.

[0084] Previous methods for the high temperature synthesis ofnanoparticles use air-free conditions due to the combustive nature ofthe reaction precursors. However, the methods of the invention providefor exposing the reactants to the oxygen reaction promoter in such a wayso as to reduce or eliminate this combustion hazard.

[0085] VI. Methods of Producing Nanoparticles

[0086] The methods described herein find utility in producing a varietyof nanoparticles, including metal-chalcogen nanoparticles such as CdSe,CdTe, CdS, ZnS, ZnSe, and so forth.

[0087] One embodiment of the invention is a method of producingnanoparticles comprising: (a) mixing a first precursor and at least onecoordinating solvent to form a first mixture; (b) exposing the firstmixture to a reaction promoter selected from the group consisting ofoxygen and a reducing agent; (c) heating the first mixture to atemperature that is sufficiently high to form nanoparticles (nucleationcrystals) when a second precursor is added; (d) introducing a secondprecursor into the first mixture to form a second mixture therebyresulting in the formation of a plurality of nanoparticles; and (e)cooling the second mixture to stop further growth of the nanoparticles.

[0088] In one embodiment, the reaction promoter is a reducing agent andthe exposing step can involve adding a chemical reducing agent to themixture or the mixture can be exposed to an appropriate electrodereducing agent. In another embodiment, the exposure step involvesdirectly exposing the mixture to a source of oxygen. The oxygen can befrom an external source or it can be created in situ. In yet anotherembodiment, the exposing step involves exposing a coordinating solventto a source of oxygen and then adding this exposed coordinating solventto the mixture.

[0089] The mixing step is typically conducted at an elevated temperatureor the reaction mixture is heated to an elevated temperature whilemixing. This elevated temperature is commonly within the range of about150 to 350° C. In addition, the mixing step can be conducted in a vesselthat is evacuated and filled and/or flushed with an inert gas such asnitrogen. The filling can be periodic or the filling can occur, followedby continuous flushing for a set period of time. The mixing step caninvolve a cooling step prior to exposure to the reaction promoter, forexample, cooling to a temperature within the range of about 50 to 150°C., typically about 100° C.

[0090] The exposing step was described above with regard to the reactionpromoter, and can be conducted either at an elevated temperature (e.g.,150 to 350° C.) or at a reduced temperature (e.g., 50 to 1 50° C.). Inaddition, the reaction promoter can be heated to a temperature such aswithin the range of about 50 to 150° C., prior to being added to themixture.

[0091] The heating step is done at a temperature that is sufficient toinduce temporally discrete homogeneous nucleation, which results in theformation of a monodisperse population of individual nanoparticles.Typically, this heating step achieves a temperature within the range ofabout 150-350° C., more preferably within the range of about 250-350° C.

[0092] It is understood, however, that the above ranges are merelyexemplary and are not intended to be limiting in any manner as theactual temperature ranges may vary, dependent upon the relativestability of the reaction promoter, precursors, ligands and coordinatingsolvents.

[0093] The introducing step may be an injection step, which typicallyinvolves applying pressure to the second precursor so that a fluidstream can be injected into the heated mixture. Pressure can be appliedin numerous ways, for example by means of a pressurized inert gas, asyringe, a pumping means, and so forth, as well as combinations thereof.The resulting mixture may be heated so as to maintain the elevatedtemperature. Thus, the introducing step is conducted at a temperaturewithin the range of about 150-350° C., more preferably within the rangeof about 250-270° C. The introducing step can be carried out in onerapid step or slowly over time.

[0094] The secondary precursor is typically combined with an appropriatecoordinating solvent to form a solution for use in the method of theinvention. This coordinating solvent may be the same or different fromthat used in combination with the first precursor.

[0095] The cooling step typically achieves a temperature within therange of about 50-150° C., more preferably within the range of about90-110° C. However, the actual temperature range may vary, dependentupon the relative stability of the reaction promoter, precursors,ligands and coordinating solvents.

[0096] Size distribution during the growth stage of the nanoparticlescan be approximated by monitoring the emission of a particle sampling.

[0097] Exemplary embodiments are set forth below, as well as in theexamples. [00871 In one exemplary embodiment, nanoparticles are producedby a method where the first precursor is exposed to the reactionpromoter, oxygen. A cadmium solution is prepared by first dissolvinganhydrous cadmium acetate in TOP, and this mixture is then mixed withTOPO, TDPA and additional TOP. Dry air is then injected into thereaction vessel so as to expose the mixture to oxygen. The duration ofexposure can be from 1-10 minutes, or longer. Heating is then done for asufficient time and temperature so as to insure formation ofnanoparticles when the second precursor is added, for example, heatingto 270° C. A selenium solution, previously prepared by dissolving Se inTOP, is then introduced by injection into the cadmium solution, thusforming CdSe nanoparticles. The reaction is stopped by cooling.

[0098] In another exemplary embodiment, nanoparticles are produced by amethod where a coordinating solvent is exposed to oxygen and then addedto the first precursor. A cadmium solution is prepared by firstdissolving anhydrous cadmium acetate in TOP, and this mixture is thenmixed with TOPO, TDPA and additional TOP. The mixture is heated to atemperature sufficiently high to insure formation of nanoparticles whenthe second precursor is added, and the elevated temperature maintained.In a separate container, TOP is heated and exposed to air. The durationof exposure can be for a period of between 10 minutes and 48 hours,preferably between 30 minutes and 24 hours, and even more preferably 50minutes to 2 hours. The air-exposed TOP is then added to the cadmiumsolution. A selenium solution, previously prepared by dissolving Se inTOP, is then introduced by injection into the cadmium solution, thusforming CdSe nanoparticles. The reaction is stopped by cooling.

[0099] Another exemplary embodiment, pertains to the production ofnanoparticles by a method where the first precursor is exposed to achemical reducing agent reaction promoter. A cadmium solution isprepared by first dissolving anhydrous cadmium acetate in TOP, and thismixture is then mixed with TOPO and TDPA. A chemical reducing agent suchas dicyclohexylphosphine, diphenylphosphine or hydroquinone is thenadded to the mixture, and the mixture heated to a temperaturesufficiently high to insure formation of nanoparticles when the secondprecursor is added. A selenium solution, previously prepared bydissolving Se in TOP, is then introduced by injection into the cadmiumsolution, thus forming CdSe nanoparticles. The reaction is stopped bycooling.

[0100] Another exemplary embodiment, pertains to the production ofnanoparticles by a method wherein the addition of a reaction promoter isused to induce nucleation. A precursor solution is prepared by firstdissolving anhydrous cadmium acetate in TOP, and this mixture is thenmixed with TOPO and TDPA and a TOP solution of Se. The mixture is heatedto a temperature sufficiently high to insure formation of nanoparticleswhen the promoter is added. Nucleation and subsequent growth ofnanoparticles is then induced by introduction of a suitable reactionpromoter. For example, a chemical reducing agent reaction promoter suchas dicyclohexylphosphine, diphenylphosphine or hydroquinone is thenintroduced by injection into the mixture, thus forming CdSenanoparticles. The reaction is stopped by cooling.

[0101] Another exemplary embodiment, illustrated in FIG. 8, pertains tothe production of nanoparticles by a method where the first precursor isexposed to an electrode reducing agent reaction promoter. The system 10includes a reaction vessel 12 having first 14 and second 16 compartmentsseparated by an ion permeable barrier 18. A cadmium solution is preparedby first dissolving anhydrous cadmium acetate in TOP, and this mixtureis then mixed with TOPO and TDPA. The resulting mixture 20 is placed incompartment 16. An electrochemical system reaction promoter, such as aplatinum cathode 22 is then immersed in mixture 20. A platinum anode 24would be set up immersed in a solution containing an oxidizablecomponent (e.g., iodide) 26 in compartment 14 and an appropriate powersupply 28 would be set up outside the reaction vessel, and in electricalcommunication with cathode 22 and anode 24 through leads 32 and 34,respectively. Optionally, magnetic stirring bars 30 can be placed incompartments 14 and 16. The mixture is heated to a temperaturesufficiently high to insure formation of nanoparticles when the secondprecursor is added at an appropriate potential. A selenium solution,previously prepared by dissolving Se in TOP, is then injected into thecadmium solution 20 in compartment 16. A negative potential is appliedat the cathode 22 to induce formation of nanoparticles. The reaction isstopped by cooling, and/or by removing the potential.

[0102] VII. Methods of Forming Nanoparticle Shells

[0103] The surface of the semiconductor nanoparticle can be modified toenhance the efficiency of the emissions, by adding an inorganic layer orshell. The overcoating layer can be particularly useful since surfacedefects on the semiconductor nanoparticle can result in traps forelectrons or holes that degrade the electrical and optical properties ofthe nanoparticle. An insulating layer at the surface of the nanoparticleprovides an atomically abrupt jump in the chemical potential at theinterface that eliminates energy states that can serve as traps for theelectrons and holes. This results in higher efficiency in theluminescent process.

[0104] The nanoparticles produced by the methods described herein can beprovided with a shell by any method known in the art. See for example,Dabbousi et al., J. Phys. Chem. B 101:9463 (1997), Hines et al., J.Phys. Chem. 100:468-471 (1996), Peng et al., J. Am. Chem. Soc.119:7019-7029 (1997), and Kuno et al., J. Phys. Chem. 106:9869 (1997).In addition, the nanoparticles of the invention can also be providedwith a shell using the reaction promoter-based method described herein.In fact, the reaction promoter-based methods of the invention findutility in nanoparticle shell procedures for both nanoparticle coresproduced by the methods described herein as well as nanoparticle coresproduced by other methods.

[0105] The shell can have a thickness within the range of about 1-100nm, and is preferably within the range of about 2-10 nm thick.

[0106] Suitable materials for the inorganic shell layer includesemiconductor materials having a higher bandgap energy than thesemiconductor nanoparticle core. In addition to having a bandgap energygreater than the core, suitable materials for the shell should have goodconduction and valence band offset with respect to the core. Thus, theconduction band is desirably higher and the valence band is desirablylower than those of the core. For a semiconductor nanoparticle core thatemits energy in the visible (e.g., CdS, CdSe, CdTe, ZnSe, ZnTe, GaP,GaAs) or near IR (e.g., InP, InAs, InSb, PbS, PbSe) range, materialsthat have a bandgap energy in the ultraviolet regions may be used.Exemplary materials include ZnS, GaN, and magnesium chalcogenides, e.g.,MgS, MgSe, and MgTe. For a semiconductor nanoparticle core that emits inthe near IR range, materials having a bandgap energy in the visiblerange, such as CdS or CdSe, may also be used.

[0107] In addition, a passivation layer of the desired thickness canalso be easily introduced onto semiconductor nanoparticles of theinvention by introducing appropriate solvents and/or surfactants duringthe nanoparticle manufacture. For example, semiconductive nanoparticles,as manufactured by the methods described herein, can be provided with awater-insoluble organic overcoat that has an affinity for thesemiconductive core material. This coating will typically be apassivating layer produced by one or more coordinating solvents asdescribed above, e.g., hexyldecylamine, TOPO, TOP, TBP, and so forth; orproduced by one or more hydrophobic surfactants such as, by way ofexample, octanethiol, dodecanethiol, dodecylamine, tetraoctylammoniumbromide, and so forth, as well as combinations thereof.

[0108] Further, as noted above, the nanoparticle shell can be producedby the reaction promoter-based methods of the invention. Accordingly, anembodiment of the invention is a method of producing a nanoparticleshell comprising: producing nanoparticles using steps (a) though (e)described above in Section VI; (a′) mixing the nanoparticles with atleast one coordinating solvent to form a third mixture; (b′) heating thethird mixture to a temperature that is sufficiently high to form a shellon the nanoparticles when third and fourth precursors are added; (c′)introducing third and fourth precursors into the third mixture to form afourth mixture thereby resulting in the formation of shells on aplurality of nanoparticles; and (d′) cooling the third mixture to stopfurther growth of the shell; wherein the method further comprisesexposing the third or fourth mixture to a reaction promoter selectedfrom the group consisting of oxygen and a reducing agent. Exposure tothe reaction promoter can occur at several stages during the shellproduction method. For example, the third mixture can be exposed to thereaction promoter after step (a′), the heated third mixture can beexposed to the reaction promoter after step (b′), or the fourth mixturecan be exposed to the reaction promoter in step (c′). In a preferredembodiment, the third mixture is exposed to the reaction promoter afterstep (a′).

[0109] As noted above, the semiconductor materials used in the shellpreferably have a higher bandgap energy than the semiconductornanoparticle core. Therefore, the precursors used in steps (d) (“first”and “second” precursors) are preferably different than the precursorsused in step (c′) (“third” and “fourth” precursors). However, the ligand(if used) and coordinating solvents used in step (a′) can be the same ordifferent from those used in step (a).

[0110] In addition, the shell producing method of the invention alsofinds utility for nanoparticle cores produced by any method known in theart. Accordingly, another embodiment of the invention is a method ofproducing nanoparticle shells comprising (a) mixing nanoparticles withat least one coordinating solvent to form a first mixture; (b) heatingthe first mixture to a temperature that is sufficiently high to form ashell on the nanoparticles when first and second precursors are added;(c) introducing first and second precursors into the first mixture toform a second mixture thereby resulting in the formation of shells on aplurality of nanoparticles; and (d) cooling the second mixture to stopfurther growth of the shell; wherein the method further comprisesexposing the first or second mixture to a reaction promoter selectedfrom the group consisting of oxygen and a reducing agent. As notedabove, exposure to the reaction promoter can occur at several stagesduring the shell production method. For example, the first mixture canbe exposed to the reaction promoter after step (a), the heated firstmixture can be exposed to the reaction promoter after step (b), or thesecond mixture can be exposed to the reaction promoter in step (c). In apreferred embodiment, the first mixture is exposed to the reactionpromoter after step (a).

[0111] VIII. Shell Additives

[0112] The methods of the invention also find utility in nanoparticlesas described in commonly owned, co-pending U.S. patent application Ser.No. 10/198,635, filed on Jul. 17, 2002, by Treadway et al., thedisclosure of which in incorporated herein in its entirety, for bothnanoparticles produced by the methods described herein or produced bystate of the art methods.

[0113] Accordingly, an embodiment of the invention is a method ofproducing nanoparticles comprising: producing nanoparticles using steps(a) though (e) described above or produced by any state of the artmethod; and (a′) mixing the nanoparticles with an additive or additiveprecursor, an optional ligand and at least one coordinating solvent toform a third mixture; (b′) exposing the third mixture to a reactionpromoter selected from the group consisting of oxygen and a reducingagent; (c′) heating the third mixture to a temperature that issufficiently high to form a shell on the nanoparticles when third andfourth precursors are added; (d′) introducing third and fourthprecursors into the third mixture to form a fourth mixture therebyresulting in the formation of shells on a plurality of nanoparticles;and (e′) cooling the fourth mixture to stop further growth of the shell.The additive or additive precursor can be any inorganic material that issuitable for use in the manufacture of semiconductor nanoparticles, suchas those described herein.

[0114] The shell-forming aspect of the inventions as described herein isillustrated as follows. InAs nanoparticles are produced by the methodsdescribed herein or by methods that are well known in the art. Thenanoparticles are then mixed with an additive precursor (e.g., a sourceof In⁺³), an optional ligand and at least one coordinating solvent toform a mixture. The mixture is then exposed to a reaction promoter suchas DPP. The mixture is heated to the appropriate temperature and firstand second precursors (e.g., a source of Cd²⁺ and Se⁰) are introduced byinjection into the mixture to form a CdSe shell. The mixture is thencooled to stop further growth of the shell. Since the Cd⁺² must bereduced to Cd⁰ and the In⁺³ must be reduced to In⁰, at least 5equivalents of the reaction promoter would be optimal, depending, atleast in part, on the relative amounts of additive (In⁺³) and firstprecursor (Cd²⁺).

[0115] IX. Methods of Overcoating Nanoparticles

[0116] The nanoparticles of the invention may also be provided with anorganic coating. Suitable organic materials include agaroses; cellulose;epoxies; and polymers such as polyacrylamide, polyacrylate,poly-diacetylene, polyether, polyethylene, polyimidazole, polyimide,polypeptides, polyphosphate, polyphenylene-vinylene, polypyrrole,polysaccharide, polystyrene, polysulfone, polythiophene, and polyvinyl.The coating can also be a material such as silica glass; silica gel;siloxane; and the like.

[0117] Therefore, the invention also encompasses a method of producingcoated nanoparticles comprising: producing nanoparticles, wherein thenanoparticle core and/or shell is produced by the methods of theinvention; and mixing the nanoparticles with an organic compound havingaffinity for the nanoparticle surface, whereby the organic compounddisplaces the coordinating solvent to form a coating on the nanoparticlesurface. The organic coating step is preferably conducted at atemperature within the range of about 50 to 350° C., preferably withinthe range of about 150 to 250° C. The actual temperature range of thecoating step may vary, dependent upon the relative stability of thereaction promoter, precursors, ligands, coordinating solvents andoverlayer composition.

EXAMPLES

[0118] The practice of the present invention will employ, unlessotherwise indicated, conventional techniques of synthetic inorganic,organic chemistry, chemical engineering, and the like, which are withinthe skill of the art. Such techniques are explained fully in theliterature. See, for example, Kirk-Othmer's Encyclopedia of ChemicalTechnology; House's Modern Synthetic Reactions; and the ChemicalEngineer's Handbook.

[0119] The following examples are put forth so as to provide those ofordinary skill in the art with a complete disclosure and description ofhow to make and use the compositions and methods of the invention.Efforts have been made to ensure accuracy with respect to numbers (e.g.,amounts, temperature, etc.) but some experimental error and deviationsshould, of course, be allowed for. Unless indicated otherwise, parts areparts by weight, temperature is degrees centigrade and pressure is at ornear atmospheric. All components were obtained commercially unlessotherwise indicated.

Materials

[0120] In all the examples which follow, materials were obtained asfollows, unless otherwise indicated: tri-n-octylphosphine oxide(TOPO, >97% purity) was from Fluka; tri-n-octylphosphine (TOP, 90%purity) was from Alfa Aesar; selenium (99.99% purity) anddicyclohexylphosphine (98% purity) were from Strem; diphenylphosphine(DPP, >90% purity, 99.9% by certificate of analysis) and hydroquinone(+99% purity) were obtained from Aldrich; and anhydrous cadmium acetate(99% purity) was from Prochem. Tetradecylphosphonic acid (TDPA, 98%purity) was either obtained from Alfa or synthesized using methods wellknown in the art (Kosolapoff, et al., J. Am. Chem. Soc. 67:1180-1182(1945). All reagents were used as received without further purification.

Example 1 Reactions with Phosphines Added

[0121] TOPO (6.0 g) and TDPA (0.577 g) were combined in a three-neckround bottom flask equipped with a stir bar, a thermocouple attached toa temperature controller unit, and a condenser connected to anitrogen/vacuum manifold. The third neck was sealed with a septum. Theatmosphere inside the reactor was evacuated once and refilled with drynitrogen. Inside an inert atmosphere glove box, a cadmium precursorsolution (0.5 m) was prepared by combining cadmium acetate (21.6 g) andTOP (166 g) and allowing the mixture to stir for˜24 hours until fullydissolved. An aliquot (2.03 g) of this solution was diluted with TOP(3.6 mL) and injected via syringe into the reaction vessel. A 16-gaugeneedle was inserted into the septum of the reaction vessel so that thevessel could be continuously flushed with nitrogen for approximately 10min as the reaction was heated to 250° C. The needle was removed andheating continued to 260° C. Heating was continued at 260-270° C. for 10min. The reaction was cooled to 100° C. and vacuum degassed for 30 minbefore the vessel was returned to a nitrogen atmosphere. An amount ofphosphine (table below) was added as one portion via syringe. Theselenium stock solution was prepared by combining selenium (3.2 g) andTOP (66.0 g) inside the inert atmosphere glove box. The temperaturecontroller attached to the reaction vessel was set to 290° C. and at the270° C. mark, selenium stock (1.4 mL) was rapidly injected to inducenanoparticle formation. Small aliquots were removed periodically fromthe stirring reaction and diluted in hexane so that emission spectracould be obtained as a function of reaction time. TABLE 1 AdditiveProportion Amount Dicyclohexylphosphine 1 × 415 μL Diphenylphosphine0.25 × 90 μL Diphenylphosphine 1 × 360 μL Diphenylphosphine 5 × 1.8 mLDiphenylphosphine 10 × 3.6 mL

Example 2 Reactions with Hydroquinone Added

[0122] These reactions were carried out as described in Example 1, withthe following modifications. No phosphine-based reaction promoter wasadded to these reactions. Instead hydroquinone (0.226 g) was combinedwith the TOPO and TDPA solids prior to the nitrogen flush of the reactorin the first step.

Example 3 Reactions Demonstrating the Use of Air as a Reagent

[0123] In this example, TOP was obtained from Fluka and used asreceived. A solution of Se was prepared by dissolving Se (3.16 g) in TOP(33.2 g) (TOPSe). Separately, a cadmium precursor stock solution wasprepared by dissolving anhydrous cadmium acetate (6.15 g) in TOP to afinal volume of 40 mL (cadmium stock solution). In each of three roundbottom flasks, TOPO (5.0 g) was combined with cadmium stock solution(1.4 mL), TDPA (0.52 g), and TOP (1.1 mL) and heated to 250° C. whilecontinuously flushing the vessel with N₂. Once the temperature reached250° C., the nitrogen flush was halted and the temperature was increasedto 270° C. This temperature was maintained for 20 min and the solutionswere cooled to 100° C. Using a large-bore needle, dry air was directedinto each of two flasks at a rate of 200 ml/min for a duration of 1minute or 10 minutes. A third flask received dry nitrogen for 10 minutesat the same flow rate. Stirring of the solution was maintainedthroughout. After the exposure period, the flasks were evacuated andrefilled with dry nitrogen. This was repeated once. The flasks were thenreheated to 270° C. and an aliquot of the previously prepared TOPSesolution (1.4 mL) was rapidly injected. The reaction temperature wasmaintained at 270° C. while small samples were periodically removed.Reactions were stopped by cooling to 100° C.

[0124] The time period between injection of TOPSe and the firstappearance of color was noted. This “induction time” is related to thereactivity of the solution. Yields of the reactions were determined bythe peak band-edge absorbance, normalized for particle size. Results arepresented in Table 2 and FIG. 6. TABLE 2 Condition, time Induction time,Relative particle Peak absorbance of exposure to air seconds yieldwavelength, nm 10 minutes dry 45 1 582 nitrogen 1 minute dry air 6 2.8561 10 minutes dry air 2 18.7 510

[0125] This data indicates that exposure to air resulted in a higheryield than the control conditions, and that the yield is modulated bythe length of the air exposure period. Further, it can be seen that thetime course evolution of particle size and particle size distribution,is also controllable by the method of the invention. This shows that thereaction can be tuned to achieve a target size while still maintaining ahigh yield and good particle size uniformity.

Example 4

[0126] Cadmium acetate/TOP stock and TOPSe were prepared as in Example3. In a round bottom flask, 2.5 g TOPO was combined with 0.703 mLcadmium acetate/TOP stock, 0.26 g tetradecylphosphonic acid and 0.55 mLTOP and heated to 250° C. while sparging with N₂. Once the temperaturereached 250° C., sparging was stopped, the temperature was increased to270° C. and held at this temperature for 20 minutes. The solution wascooled to 100° C. One neck of the flask was opened and dry compressedair was directed into the stirring solution for 10 minutes. After theexposure period, the flask was evacuated and refilled with dry nitrogen.This was repeated two more times. The flask was then reheated to 240° C.and 0.7 mL of TOPSe was rapidly injected. After 15 seconds, the reactionwas stopped by injecting 5 mL TOP and removing the heat source. Analiquot was removed and measurement showed the band-edge absorbance peakat 448 nm with the luminescence peak at 471 nm with 31 nm FWHM.

[0127] This data illustrates the ability to independently control thefundamental aspects of the crystal growth process, thereby enabling thehigh yield synthesis of very small CdSe nanoparticles.

Example 5 Reactions Marking Use of Pre-Air Treatment Reagents

[0128] In this example, TOP was obtained from Fluka and used asreceived. The cadmium precursor and TOPSe were prepared as in Example 3.In each of two round bottom flasks, TOPO (3.0 g) was combined withcadmium stock (0.76 mL), TDPA (0.282 g), and TOP (1.24 mL) and heated to250° C. while continuously flushing with dry nitrogen. Once thetemperature reached 250° C., nitrogen-flushing was halted, and thetemperature was increased to 270° C. and held at this temperature. In aseparate nitrogen-blanketed flask, TOP (˜5 mL) was heated to 100° C.Once the temperature reached 100° C., the flask was opened to air andheld at 100° C., while stirring, for 50 minutes. The flask was thenclosed and evacuated, followed by refilling with nitrogen. Thispurge/refill was repeated one more time. To one flask containing thecadmium solution (at 270° C.) was added air-exposed TOP (1 mL). To theother cadmium-containing flask (at 270° C.) was added unexposed TOP (1mL). Using a syringe, an aliquot of TOPSe stock (0.71 mL) was rapidlyinjected into each flask. The temperature was maintained at 270° C.while small samples were periodically removed. Reactions were stopped bycooling to 100° C.

[0129] The time period between injection of TOPSe and the firstappearance of color was noted. Yield of the reactions was determined bythe peak band-edge absorbance, normalized for particle size. Results arepresented in Table 3 and FIG. 7.

[0130] This data indicates that exposure of TOP to air generates amaterial that can be added to a reaction mix, resulting in higher yieldand with useful modulation of particle size and particle sizedistribution. TABLE 3 Exposure time of Induction time, Relative particlePeak absorbance TOP to air seconds yield wavelength, nm None 18 1 583 50minutes 4 2.875 579

[0131] All patents, publications, and other published documentsmentioned or referred to herein are incorporated by reference in theirentireties.

[0132] It is to be understood that while the invention has beendescribed in conjunction with the preferred specific embodimentsthereof, that the foregoing description as well as the examples thatfollow, are intended to illustrate and not limit the scope of theinvention. It should be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the invention, and further that otheraspects, advantages and modifications will be apparent to those skilledin the art to which the invention pertains.

We claim:
 1. A method of producing nanoparticles comprising: (a) mixinga first precursor and at least one coordinating solvent to form a firstmixture; (b) exposing the first mixture to a reaction promoter selectedfrom the group consisting of oxygen and a reducing agent; (c) heatingthe first mixture to a temperature that is sufficiently high to formnanoparticles when a second precursor is added; (d) introducing a secondprecursor into the first mixture to form a second mixture therebyresulting in the formation of a plurality of nanoparticles; and (e)cooling the second mixture to stop further growth of the nanoparticles.2. The method of claim 1 wherein the reaction promoter is a reducingagent.
 3. The method of claim 2 wherein the exposing step comprisesadding the reducing agent to the first mixture.
 4. The method of claim 2wherein the reducing agent is a chemical reducing agent selected fromthe group consisting of tertiary, secondary, and primary phosphines;amines; hydrazines; hydroxyphenyl compounds; hydrogen; hydrides; metals;boranes; aldehydes; alcohols; thiols; reducing halides; andpolyfunctional reductants.
 5. The method of claim 2 wherein the reducingagent is a cathode.
 6. The method of claim 5 wherein the cathode is madeof a material selected from the group consisting of platinum, silver,and carbon.
 7. The method of claim 1 wherein the reaction promoter isoxygen.
 8. The method of claim 7 wherein the exposing step comprisesdirectly exposing the first mixture to a source of oxygen.
 9. The methodof claim 7 wherein the oxygen is formed in situ.
 10. The method of claim9 wherein the oxygen is formed by a redox reaction.
 11. The method ofclaim 10 wherein the exposing step comprises adding a reducing agentselected from the group consisting of tertiary, secondary, and primaryphosphines; amines; hydrazines; hydroxyphenyl compounds; hydrogen;hydrides; metals; boranes; aldehydes; alcohols; thiols; reducinghalides; and polyfunctional reductants.
 12. The method of claim 7wherein the exposing step comprises exposing a coordinating solvent to asource of oxygen and adding the exposed coordinating solvent to themixture.
 13. The method of claim 1 wherein the coordinating solvent isselected from the group consisting of amines, alkyl phosphines, alkylphosphine oxides, fatty acids, ethers, furans, phospho-acids, pyridines,alkenes, alkynes and combinations thereof.
 14. The method of claim 13wherein the coordinating solvent is pure.
 15. The method of claim 1wherein the mixing step further includes mixing a ligand with the firstprecursor and coordinating solvent.
 16. The method of claim 15 whereinthe ligand is selected from the group consisting of phospho-acids,carboxylic acids, amines, amides, alcohols, ethers, alkenes, andalkynes.
 17. The method of claim 16 wherein the ligand is a phosphonicacid.
 18. The method of claim 17 wherein the phosphonic acid is selectedfrom the group consisting of hexylphosphonic acid andtetradecylphosphonic acid.
 19. The method of claim 1 wherein thecoordinating solvent is a mixture of a non-coordinating solvent and aligand.
 20. The method of claim 19 wherein the non-coordinating solventis an alkane.
 21. The method of claim 19 wherein the ligand is selectedfrom the group consisting of phospho-acids, carboxylic acids, amines,amides, alcohols, ethers, alkenes, and alkynes.
 22. The method of claim1 wherein the nanoparticles are semiconductive.
 23. The method of claim22 wherein the first precursor is selected from the group consisting ofGroup 2, 12, 13 and 14 element-containing compounds.
 24. The method ofclaim 22 wherein the second precursor is selected from the groupconsisting of Groups 14, 15 and 16 element-containing compounds.
 25. Themethod of claim 1 which further comprises producing a nanoparticleshell.
 26. The method of claim 25 wherein the shell producing stepcomprises: (a′) mixing the nanoparticles with at least one coordinatingsolvent to form a third mixture; (b′) heating the third mixture to atemperature that is sufficiently high to form a shell on thenanoparticles when third and fourth precursors are added; (c′)introducing third and fourth precursors into the third mixture to form afourth mixture thereby resulting in the formation of shells on aplurality of nanoparticles; and (d′) cooling the third mixture to stopfurther growth of the shell; wherein the method further comprisesexposing the third or fourth mixture to a reaction promoter selectedfrom the group consisting of oxygen and a reducing agent.
 27. The methodof claim 26 wherein an additive or additive precursor is included instep (a′).
 28. A method of producing nanoparticle shells comprising: (a)mixing nanoparticles with at least one coordinating solvent to form afirst mixture; (b) heating the first mixture to a temperature that issufficiently high to form a shell on the nanoparticles when first andsecond precursors are added; (c) introducing first and second precursorsinto the first mixture to form a second mixture thereby resulting in theformation of shells on a plurality of nanoparticles; and (d) cooling thesecond mixture to stop further growth of the shell; wherein the methodfurther comprises exposing the first or second mixture to a reactionpromoter selected from the group consisting of oxygen and a reducingagent.
 29. The method of claim 28 wherein the reaction promoter is areducing agent.
 30. The method of claim 29 wherein the exposing stepcomprises adding the reducing agent to the mixture.
 31. The method ofclaim 29 wherein the reducing agent is a chemical reducing agentselected from the group consisting of tertiary, secondary, and primaryphosphines; amines; hydrazines; hydroxyphenyl compounds; hydrogen;hydrides; metals; boranes; aldehydes; alcohols; thiols; reducinghalides; and polyfunctional reductants.
 32. The method of claim 29wherein the reducing agent is a cathode.
 33. The method of claim 32wherein the cathode is made of a material selected from the groupconsisting of platinum, silver and carbon.
 34. The method of claim 28wherein the reaction promoter is oxygen.
 35. The method of claim 34wherein the exposing step comprises directly exposing the mixture to asource of oxygen.
 36. The method of claim 34 wherein the oxygen isformed in situ.
 37. The method of claim 36 wherein the oxygen is formedby a redox reaction.
 38. The method of claim 37 wherein the exposingstep comprises adding a reducing agent selected from the groupconsisting of tertiary, secondary, and primary phosphines; amines;hydrazines; hydroxyphenyl compounds; hydrogen; hydrides; metals;boranes; aldehydes; alcohols; thiols; reducing halides; andpolyfunctional reductants.
 39. The method of claim 34 wherein theexposing step comprises exposing a coordinating solvent to a source ofoxygen and adding the exposed coordinating solvent to the mixture. 40.The method of claim 28 wherein the nanoparticles are semiconductive. 41.The method of claim 28 wherein an additive or additive precursor isincluded in step (a).
 42. A method of producing semiconductivenanoparticles having a valency “n”, comprising: (a) mixing a firstprecursor having a valency “c” and at least one coordinating solvent toform a first mixture; (b) exposing the first mixture to a reactionpromoter wherein the reaction promoter converts the valency of the firstprecursor to a valency “a”; (c) heating the first mixture to atemperature that is sufficiently high to form nanoparticles when asecond precursor is added; (d) introducing a second precursor into thefirst mixture to form a second mixture thereby resulting in theformation of a plurality of nanoparticles; wherein the second precursorhas a valency “b”, and wherein a+b=n and c+b≠n; and (e) cooling thesecond mixture to stop further growth of the nanoparticles.
 43. Themethod of claim 42 wherein the reaction promoter is a reducing agent.44. The method of claim 43 wherein the reducing agent is a chemicalreducing agent selected from the group consisting of tertiary,secondary, and primary phosphines; amines; hydrazines; hydroxyphenylcompounds; hydrogen; hydrides; metals; boranes; aldehydes; alcohols;thiols; reducing halides; and polyfunctional reductants.
 45. The methodof claim 44 wherein the coordinating solvent is pure.
 46. The method ofclaim 43 wherein the reaction promoter is a cathode.
 47. The method ofclaim 46 wherein the cathode is made of a material selected from thegroup consisting of platinum, silver and carbon.
 48. The method of claim42 wherein the reaction promoter is a source of oxygen.
 49. The methodof claim 42 wherein the reaction promoter is oxygen that is formed insitu by a redox reaction.
 50. The method of claim 49 wherein theexposing step comprises adding a reducing agent selected from the groupconsisting of tertiary, secondary, and primary phosphines; amines;hydrazines; hydroxyphenyl compounds; hydrogen; hydrides; metals;boranes; aldehydes; alcohols; thiols; reducing halides; andpolyfunctional reductants.
 51. A method of producing nanoparticles,comprising: (a) mixing first and second precursors with at least onecoordinating solvent to form a first mixture; (b) heating the firstmixture to a temperature that is sufficiently high to form nanoparticleswhen a reaction promoter is added; (c) exposing the first mixture to areaction promoter, said reaction promoter being selected from the groupconsisting of oxygen and a reducing agent, to form a second mixturethereby resulting in the formation of a plurality of nanoparticles; and(d) cooling the second mixture to stop further growth of thenanoparticles.
 52. The method of claim 51 wherein the reaction promoteris a reducing agent selected from the group consisting of chemicalreducing agents and cathodes.
 53. The method of claim 51 wherein thereaction promoter is oxygen, and the oxygen is added by exposing thefirst mixture to a source of oxygen.
 54. The method of claim 51 whereinthe reaction promoter is oxygen, and the oxygen is added to the firstmixture by an in situ redox reaction.